The arrest of a 25-year-old man for alleged terror offences is linked to the investigation into threats to kill the Prime Minister, police sources said.
The man, who is believed to be white, is the fourth person from Blackburn, Lancashire, to be arrested on suspicion of committing offences under the Terrorism Act 2006 in the last two weeks.
Three men, aged 21, 22 and 23, were arrested on August 14. Two were held at Manchester Airport as they were about to board a flight to Finland, and the third man was detained in Accrington, Lancashire.
The trio are linked to an investigation into threats to kill the Prime Minister, according to police sources.
No plot was in place but there were calls for the death of Gordon Brown on an extremist jihad website in January.
The internet hosts many websites from groups and individuals claiming allegiance to al Qaida and Osama bin Laden, who routinely issue warnings and threats to political leaders in the UK and the US over the West's involvement in the Middle East.
Police have until Thursday to charge the three men, release them or seek an extension to their custody.
The 25-year-old man was arrested at 6.40am. Specialist officers were searching an address in the Whalley Range area of the town.
A police spokeswoman said: "The arrest is part of an ongoing investigation by Lancashire Constabulary and Greater Manchester Counter Terrorism Unit."
Chief Superintendent Andy Rhodes, Divisional Commander for Eastern Division of Lancashire Police, said: "This is a very sensitive time for all our communities and we are working hard to ensure everyone is kept up to date with as much information as we can give."
